1. Field of the Invention
The invention relates to methods of manufacturing semiconductor silicon bodies and somewhat more particularly to a process for producing amorphous silicon bodies via glow discharge and which are useful in fabricating solar cells.
2. Prior Art
In the manufacture of certain electrical components composed of silicon, such as solar cells, a silicon material can be used which does not have to meet as stringent requirements relative to its crystal quality and purity as is required in fabricating integrated semiconductor circuits. Because solar cells must be very economical in comparison to integrated semiconductor circuits in order to have broad application, (i.e., for miniature equipment in a watt range which is independent of a main power source or for generators in the KW-range which are also independent of a main power source) the manufacture of silicon bodies used as raw or base materials in fabricating solar cells must also be as simple and economical as possible.
Amorphous silicon (so-called a-Si) is an interesting and promising material for solar cell fabrication. Solar cells composed of this material are known, for example see U.S. Pat. No. 4,064,521 or German Offenlegungsschrift No. 27 43 141.
The method of producing amorphous silicon suggested by U.S. Pat. No. 4,064,521 comprises depositing an amorphous silicon layer having a thickness of about 1 .mu.m on a sheet steel substrate via thermal decomposition of silane (SiH.sub.4) in a low pressure plasma system. A Schottky-contact is fabricated on a surface of such layer by a subsequent vapor-deposition of a very thin metallic layer, such as a platinum layer. In order to achieve a relatively high efficiency (i.e., 5.5% has been achieved) in solar cells fabricated from such a-Si layers, it is necessary to incorporate atomic hydrogen into the silicon material. Apparently, atomic hydrogen saturates the free valences (sometimes referred to as "dangling" bonds) in amorphous silicon so that such valences or dangling bonds cannot function as recombination centers for charge carriers released by light. The incorporation of hydrogen in a-Si occurs simultaneously during the decomposition process of silane in a plasma reactor.
The method of producing amorphous silicon suggested by German Offenlegungsschrift No. 27 43 141 comprises decomposing a chlorinated or brominated silane, rather than the more expensive silane hydride (SiH.sub.4). In this process, the deposition parameters are controlled in such a manner that the resultant amorphous silicon layer contains up to about 7 atomic percent of a halogen selected from the group consisting of Cl, Br and I as well as hydrogen for saturation and/or compensation of the free valences present in a-Si. A disadvantage of this process is that when the halogen content becomes too high, the electrical properties of the resultant amorphous silicon are negatively influenced and thus, the efficiency of solar cells fabricated from such material is substantially reduced.